On the rare occasions that science fiction television series have experimented with feature-length spin-offs, the results have been varied at best. Of the five Babylon 5 films, for example, only In the Beginning proved satisfying.

In the Beginning maintained continuity with, and enriched, the story arc, while revealing unexpected aspects of characters. The same can be said of Battlestar Galactica: Razor, a telefilm that makes no compromises for viewers new to the series.

BSG is clearly cousin to Babylon 5 in its multi-seasonal plotting, but its political maturity, psychological depth, and naturalistic scripting far outclasses B5's political naivety and open portent.

Having said this, Razor is not without its faults. The razor metaphor and the motif of a hereditary lock knife are unusually crude devices in what is largely a subtle series.

These minor quibbles aside, the film's strengths certainly outweigh its shortcomings. Magnificent special effects, a coherent political allusiveness, dramatic characterisation, and a continuing interest in the themes of betrayal and redemption, all compare well with BSG's tonally darker episodes.

Execution, torture, degradation and vivisection mark Razor as a disturbingly visceral addition to the series. Fortunately, its achievement lies less in foregrounding these, than in its sophisticated treatment of time and history.

Razor is set between Season Two's episodes, The Captain's Hand and Downloaded. Its events take place in three time zones: in the narrative present, when Lee Adama is commander of the Battlestar Pegasus; in the aftermath of the Cylon's destruction of the twelve colonies as the Pegasus commences its guerilla operations; and in the final moments of the first Cylon War.

The transitions between each reaffirm BSG's preoccupation with consequence and how characters are complicated and compromised by their decisions.

The appearance of antique Cylons, Raiders and Basestars establish new associations with BSG's first incarnation. While this may appear little more than a self-conscious nod to the show's own history (fostering a rich sense of nostalgia for Larson's campy original), their inclusion reminds us that the past is omnipresent, forming and informing contemporary experience and events.

Razor invites the viewer to acknowledge the path that has led its characters and our world into its current condition.

As a consequence of its airdate, Razor also forces the audience to adopt a Janus-like perspective on the series. Just as we look back on Adama's youth and Cain's authoritarian regime aboard the Pegasus, and watch with irony the beginnings of Lee Adama's brief command of the vessel, so we look forward to reinterpret the character trajectories and events of Season Three with new insights.

The non-linearity of Razor, its coalescence of past events, current circumstances and future echoes, is contextualised by the hybrid Cylon's refrain: "All this has happened before ... and will happen again." Linear time telescopes into mythic time, making the unknown incidents of Season Four seem both more and less significant.

Nevertheless, this ambiguity and sophistication ensures that anticipation remains a constant.

· An extended cut of Battlestar Galactica: Razor is released in the UK on December 26, 2007